1. Field of the Invention
The present invention relates to a laser light irradiation apparatus which has been improved in uniformity over an irradiated surface.
2. Description of the Related Art
In recent years, there is known a technique in which a crystalline silicon film is formed on a glass substrate and a thin film transistor is formed by using the crystalline silicon film.
As a method of obtaining a crystalline silicon film, there is known a technique in which an amorphous silicon film is first formed by a plasma CVD method or the like, and irradiation of a laser light is carried out to the amorphous silicon film to transform it into the crystalline silicon film.
An annealing method with this laser light irradiation is also used for annealing the source and drain regions of a thin film transistor formed in a self-aligned manner.
Although the method with the laser light irradiation is a technique capable of obtaining high crystallinity, there is a problem that the method is disadvantageous for a treatment of a large area.
However, in the case where an active matrix type liquid crystal display device having a large area is manufactured, there are no effective methods under the present situation other than the above-mentioned method of using the laser light.
It is an object of the invention to provide a technique in which laser light irradiation to a large area can be carried out with high uniformity. Also, another object of the invention is to provide a technique in which a crystalline silicon film having a large area is obtained by using such a laser light.
Also, it is still another object of the invention to provide a technique in which various kinds of annealing with irradiation of a laser light to a semiconductor device formed on a substrate having a large area can be carried out with high uniformity.
According to the study of the present inventors, it has been apparent that a method described below is effective as a method of annealing a silicon film having a large area. This method is such that a laser light is optically transformed into a linear beam having a width of several millimeters and a length of several tens centimeters, and the irradiation with this linear laser light is carried out while the scan therewith is carried out in the width direction thereof.
According to this method, laser light irradiation to a large area can be carried out by one scan. This method is superior in operation efficiency and uniformity of irradiation effect to a conventional method in which irradiation is carried out while scan to a spot with several centimeters square is sequentially carried out.
However, this method has a problem that the unevenness of laser irradiation density in the longitudinal direction of the linear laser beam is apt to become remarkable.
It is supposed that this problem is caused since the laser light with a width of several centimeters oscillated from an oscillator is optically enlarged into the length of several tens centimeters in the longitudinal direction of the linear laser beam.
On the other hand, in the width direction of the linear laser beam, since the laser beam with a width of several centimeters is reduced into the width of several millimeters, the uniformity in the width direction does not become a serious problem.
An apparatus for the linear laser beam irradiation is schematically shown in FIG. 4. FIG. 4 shows an oscillator 101 for oscillating a KrF excimer laser light, and a lens system of lenses 102 and 103 for optically transforming the laser light oscillated from the laser oscillator 101 into a predetermined laser beam.
Further, the laser beam from the lens system comprised of the lenses 102 and 103 is inputted into homogenizers 80 and 81 for homogenizing the distribution of energy density.
Furthermore, the laser beam from these two homogenizers 80 and 81 is inputted into a lens 106 for converging the beam in the width direction of the laser light to be finally transformed into a linear one.
Also, the laser beam is enlarged in the longitudinal direction of the linear laser light by a lens 107. Although the figure does not show that the laser beam is enlarged to a large degree as compared with the original laser beam, the laser beam with a dimension of several centimeters is actually enlarged into several tens centimeters.
Further, the laser light is reflected by a mirror 108, and is converged by a lens 109, then the beam as the linear laser light is irradiated onto a surface 100 to be irradiated.
In such a structure, the homogenizers 80 and 81 control the distribution of irradiation energy density of the irradiation laser beam.
The homogenizer 80 has a function to control the distribution of irradiation energy density in the width direction of the linear laser beam. The homogenizer 81 also has a function to control the distribution of irradiation energy density in the longitudinal direction of the linear laser beam.
Such a structure is basically for the case where square or circular laser beams are formed. That is, such a structure is effective for the case where, in the final irradiation of laser beam, the components of beam pattern in the axial directions orthogonal to each other are not much different from each other. As a conventional example of such a structure, there is known a structure disclosed in U.S. Pat. No. 4,733,944. The structure disclosed in this U.S. Patent is also an example for the case where the beam pattern in the axial directions orthogonal to each other is symmetrical.
However, when irradiation of a linear laser beam is carried out, the sectional shape of the beam in the longitudinal direction thereof is remarkably different from that of the beam in the width direction thereof. Accordingly, the state of distribution control of irradiation energy density to be obtained becomes different between the longitudinal direction and the width direction.
That is, the unevenness of irradiation energy density in the width direction of the linear laser light hardly becomes a problem because the width is narrow. However, the distribution of irradiation energy density in the longitudinal direction of the linear laser light becomes a serious problem because the dimension is largely enlarged. That is, control means for the distribution of irradiation energy density should be different for the respective directions.
The present invention disclosed in the present specification has been made on the basis of the above-described knowledge. The basic structure of the present invention is characterized in that the number of homogenizers for controlling the distribution of irradiation energy density in the longitudinal direction of a linear laser light is larger than that of homogenizers for controlling the distribution of irradiation energy density in the width direction of the linear laser light.
With this structure, there is obtained a laser irradiation apparatus in which the expensive homogenizers are effectively used to obtain necessary uniformity of annealing.
According to a first aspect of the invention, as specific structure thereof is shown in FIG. 1, a laser irradiation apparatus for irradiating a linear laser light comprises a homogenizer 11, A=1 in number, corresponding to the width direction of the linear laser light, and homogenizers 12 and 13, B=2 in number, corresponding to the longitudinal direction of the linear laser light, and is characterized by A less than B.
In the above structure, the sum of A and B is an odd number.
According to another aspect of the invention, a laser irradiation apparatus for irradiating a linear laser light is characterized in that the number of homogenizers for controlling the irradiation energy density in the width direction of the linear laser light is different from that of homogenizers for controlling the irradiation energy density in the longitudinal direction of the linear laser light.
According to a still another aspect of the invention, a laser irradiation apparatus for irradiating a linear laser light is characterized in that the total number of homogenizers is an odd number.